ZnBr₂-Ph₄PI as highly efficient catalyst for cyclic carbonates synthesis from terminal epoxides and carbon dioxide

The catalyst systems composed of ZnBr₂ and different phosphonium salts were examined for solvent-free synthesis of cyclic carbonates from CO₂ and terminal epoxides under mild conditions. Among the catalysts investigated, ZnBr₂-Ph₄PI was found to be the best while those of ZnBr₂-phosphine oxide (Bu₃PO or Ph₃PO) show no catalytic effect. It is apparent that the halide ions of phosphonium salts have an essential role to play in the reaction. The catalytic activity of ZnBr₂-Ph₄PI increases with a rise of Ph₄PI to ZnBr₂ molar ratio up to 6, above which there is little change in catalytic activity. We observed that with a rise in ZnBr₂ to Ph₄PI molar ratio, there is increase in epoxide conversion but decline in TOF_PO (estimated based on the site number of Zn²⁺). The effect of water on the reaction was investigated for the first time. We found that the presence of even a trace amount of water would result in a marked decline in reactivity, and the observation provides a valid explanation for why reproducibility of results is poor among researchers so far. The influences of other parameters such as reaction temperature and CO₂ pressure on the catalytic performance of ZnBr₂-PPh₄I were also studied. It is shown that the catalyst is sensitive to reaction temperature, and a rise of reaction temperature up to 130 °C favors the formation of cyclic carbonates. We observed that activity increases with rise in CO₂ pressure and reaches a maximum at an initial CO₂ pressure of 2.5 MPa. Moreover, a plausible reaction mechanism has been proposed.